1. Field of the Invention
The present invention relates to a method of forming a fluorescent screen for a color CRT (cathode-ray tube) and, more particularly, to a method of forming a fluorescent screen for an index color CRT.
2. Description of the Prior Art
In a color CRT manufacturing process, fluorescent materials respectively for different colors, for example, a red fluorescent material, a green fluorescent material and a blue fluorescent material, are applied respectively in predetermined patterns to the inner surface of a panel 1P separate from the funnel-shaped body 1 of a color CRT, and then the panel 1P is joined to the funneled open end of the body 1 with frit as shown in FIG. 2. Ordinarily, light-absorbing stripes CS of a light-absorbing mateial such as carbon are formed between and in front of the adjacent stripes of red, green and blue fluorescent materials to improve the contrast of pictures. Accordingly, the respective effective widths of the red, green and blue fluorescent stripes are dependent on intervals between the light-absorbing CS stripes. The pattern of the light-absorbing stripes CS is formed through steps of exposing a photosensitive resin film such as a PVA-ADC film formed over the inner surface of the panel in a pattern of the color fluorescent stripes, developing the pattern of the color fluorescent stripes, applying a light-absorbing material such as carbon to the entire inner surface of the panel over the pattern of the color fluorescent stripes, and removing the pattern of the color fluorescent stripes of the photosensitive resin film. Therefore, the respective widths of the red, green and blue stripes are dependent on the condition of exposure of the corresponding portions of the photosensitive film. In a color CRT such as a Trinitron (a registered trademark of Sony Corporation) color CRT, in which an aperture grill having slits is used for separating colors from each other, the aperture grill is used as a mask for exposing the photosensitive resin film formed over the inner surface of the panel, and a single light source is moved relative to the aperture grill for three exposure cycles respectively for the red, green and blue patterns of stripes or three light sources are used individually for exposing portions respectively corresponding to the red, green and blue patterns of stripes.
However, since the beam index color CRT is not provided with any aperture grill having slits for color separation, an exposure mask having slits, for example, a flat mask formed by forming a shading pattern over the surface of an ordinary flat glass plate, is used for exposing a photosensitive resin film formed over the inner surface of the panel, and the photosensitive resin film is exposed in the foregoing exposure method in manufacturing the beam index color CRT.
However, the foregoing conventional method has problems in the accuracy of the relative position between the light source and the exposure mask. From the viewpoint of facilitating the work for forming the light-absorbing stripes CS and productivity, it is desirable to form the light-absorbing stripes CS through a single exposure cycle using a single point light source.
The respective widths of the red, green and blue fluorescent stripes vary, depending on the respective luminance efficiencies of the red, green and blue fluorescent materials, and the width of the color fluorescent stripe in the central portion of the panel and that in the peripheral portion of the same are not the same, to secure color purity in the peripheral portion of the panel For example, when a flat exposure mask having slits each of a width proportional to the width of the corresponding fluorescent stripe to be formed is used for exposing a photosensitive resin film formed over the inner surface of a panel for a beam index color CRT, the fluorescent stripe is not formed in a width proportional to the width of the corresponding slit.
Generally, the panel for a color CRT is formed in a curved plane with a curvature. Accordingly, such a flat exposure mask is unable to expose some part of the fluorescent resin film formed over the inner surface of the panel correctly in a predetermined exposure pattern, and hence an irregular exposure pattern is formed on the inner surface of the panel, or the width of the stripe of the exposure pattern varies depending on position. It is inferred from studies made by the inventors of the present invention that such a problem is attributable to incorrect correspondence between the width of the slit of the exposure mask and the width of the stripe of the exposed pattern, due to diffraction. Suppose that an exposure mask and an exposed surface (the photosensitive resin film formed over the inner surface of the panel) are spaced by a finite distance. Then, it is known that light intensity varies in a distribution curve on the exposed surface due to diffraction when the exposure light is substantially monochromatic light. Supposing that a surface of a panel is exposed by a light source emitting monochromatic light using an exposure mask having a slit and that the light source is located at a sufficiently large distance from the surface, the pattern of diffraction is dependent on the pattern factor F expressed by an expression: ##EQU2## where A is the slit width, b is the distance between the exposure mask and the exposed surface of the panel, and .lambda. is the wavelength of the light emitted by the light source. Values of the width of stripes formed on the panel were calculated by using the expression (1), in which the wavelength .lambda. and the distance b were fixed and the slit width A was varied. The calculated results are represented by a curve T.sub.0 indicated by a solid line in FIG. 11. The range of stripe width was determined so that light intensity is greater than a threshold corresponding to the half of light intensity when the exposure mask is not provided with any slit. As obvious from the curve T.sub.O, the stripe width on the panel does not vary in proportion to the slit width of the exposure mask, so that the problem mentioned above is caused.
The diffraction pattern varies, as shown in FIGS. 4 to 10, depending on exposure conditions represented by F, the slit width A, the distance b and the wavelength .lambda.. FIGS. 4 to 10 show diffraction patterns under the exposure conditions as tabulated below.
______________________________________ Slit width Distance Wavelength F A (mm) b (mm) .lambda. (.mu.) FIG. ______________________________________ 2.4 0.20 34.7 0.40 4 3.2 0.20 19.5 0.40 5 3.4 0.20 17.3 0.40 6 3.8 0.20 13.9 0.40 7 5.4 0.20 6.9 0.40 8 6.3 0.20 5.0 0.40 9 9.6 0.20 2.2 0.40 10 ______________________________________
In FIGS. 4 to 10, light intensity is measured on the vertical axis, and the distance on the panel along the direction of the slit width is measured on the horizontal axis. The diffraction pattern varies with the distance b, which varies due to the curvature of the panel, and thereby irregular exposure is caused.
Such a problem may be solved by the use of a curved exposure mask. However, a curved exposure mask is expensive and is difficult to manufacture.